This invention relates to an improved zeolite occluding material.
Zeolite is one of adsorbents that are at present widely in use for various purposes including industrial purposes. The zeolite, particularly A type zeolite, is typically represented by a sodium-A zeolite, which is expressed by a typical unit cell of Na.sub.12 (AlO.sub.2.SiO.sub.2).sub.12. (NaAlO.sub.2).delta...omega.H.sub.2 O, wherein 0.ltoreq..delta..ltoreq.1 and .omega. represents a positive number. In this unit cell, 12 sodium ions are ion exchangeable for other metal ions. The kinds of the exchange ions and the rate of exchange is determined by the effective adsorption pore diameter. However, the size of the effective adsorption pore diameter is in close relation to the crystal structure, the size of the ion species to be exchanged and the position selecting property thereof within a unit crystal lattice. In other words, among the 12 cations (sodium ions) which are exchangeable within the unit crystal lattice of zeolite, three ions are located on the face of an eight-member oxygen ring where the molecule to be adsorbed comes in and goes out and eight ions are on the face of a six-member oxygen ring while the remaining one is located on the face of a four-member oxygen ring.
Therefore, it is the size of the cations on the face of the eight-member oxygen ring that have an influence directly on the adsorbing property of the zeolite. Where a sodium A-type zeolite is employed as starting matter and the sodium ions of the zeolite are exchanged for potassium ions, the potassium ions have a preference for entering the positions on the face of the eight-member oxygen ring. The effective adsorption pore diameter of the sodium A-type zeolite is 4 .ANG.. When the potassium ions which are larger than sodium ions enter the positions, the effective adsorption pore diameter of the ion-exchanged zeolite becomes 3 .ANG..
If the ion exchange is carried out for calcium ions, calcium ions have a preference for entering the positions on the face of the six-member oxygen ring while, among the sodium ions that are to move out to keep the balance of charges, the sodium ions on the face of the eight-member oxygen ring have priority over other sodium ions in moving out. Therefore, when an ion-exchange process is carried out until all of the sodium ions disappear from the face of the eight-member oxygen ring, the effective adsorption pore diameter of the zeolite used in the ion-exchange increases and becomes 5 .ANG..
Generally, the effective adsorption pore diameter of zeolite or that of zeolite obtained through ion-exchange is nearly uniform. A molecule smaller than the effective adsorption pore diameter of the zeolite can be adsorbed by the zeolite. However, a molecule larger than that cannot be adsorbed by the zeolite through a normal process.
The position selecting property of the ion to be exchanged with the exchangeable ion contained in the zeolite and variation that takes place in adsorbing property with variation in combination of the species of ions have not been clearly known. The present inventors conducted researches into the details of these relations. As a result of these researches, it has been discovered that a zeolite having a novel adsorbing property which has hitherto been unknown can be obtained through an ion exchange process for reformation of zeolite in terms of the adsorbing power thereof carried out with the combination of ion species and the rate of exchange suitably selected. In other words, it has been discovered that, in having the exchangeable sodium ions of a sodium type zeolite gradually exchanged for calcium ions, when two or more than two calcium ions enter, sodium ions on the face of the eight-member oxygen ring move out and this makes the effective adsorbing pore diameter 5 .ANG..
On the other hand, in case where the potassium ions of a potassium type zeolite, which is obtained by exchanging the exchangeable sodium ions of a sodium type zeolite with potassium ions, or those of a potassium type zeolite obtained with a source of potassium used as material, are processed to have them gradually exchanged for bivalent metal ions, the bivalent metal ions have preference to come onto a six-member oxygen ring face. However, when the number of the bivalent metal ions is less than 4.5 per unit crystal lattice, the potassium ions on the face of the eight-member oxygen ring are not removed from there and thus the effective adsorbing pore diameter is kept at 3 .ANG..
It has been found that, in the case of zeolite in which the effective adsorbing pore diameter is 3 .ANG. with potassium ions on the face of the eight-member oxygen ring and bivalent metal ions on a part of the six-member oxygen ring face as stated in the foregoing, a molecule of diameter larger than the effective adsorbing pore diameter can be adsorbed to the zeolite at a relatively low temperature and at a low pressure; and that the adsorbed molecule will not be desorbed even when the zeolite is brought back into ordinary desorbing condition. Namely, the zeolite has an occluding property. This indicates that the potassium ions on the face of the eight-member oxygen ring are made to be readily movable by the influence of the bivalent ions received in exchange. Such a movable state of the potassium ions on the face of the eight-member oxygen ring is believed to be dependent upon the number of the exchanged bivalent metal ions on the face of the six-member oxygen ring as well as temperature. Therefore, the molecule which is occluded in the zeolite can be released from an occluded state by raising the temperature of the zeolite. It is also possible that the occluding quantity and adsorbing and desorbing temperature can be adjusted by varying the number of the bivalent metal ions to be exchanged. In a practical application, the exchangeable cations of the zeolite do not have to be limited to potassium and bivalent metal ions but it is also permissible to have sodium ions on a part of the face of the six-member oxygen ring.